Docs: Update Unity Quickstart with Proper Minimum Supported Version (2022.3)

Our Unity Quickstart guide needs a quick update to be up to date with the minimum version of Unity prerequisite. As of [v1.15.0 (2025-02-03)](https://github.com/CesiumGS/cesium-unity/blob/main/CHANGES.md#v1150---2025-02-03) we require Unity 2022 LTS (2022.3), Unity 6, or later.

The [Quickstart guide states](https://cesium.com/learn/unity/unity-quickstart/#:~:text=The%20latest%20version%20of%20Unity%202021.3%2B%20LTS%20is%20recommended) that Unity 2021.3+ LTS is recommended.

Open the linked Unity Quickstart guide and locate the prerequisite stating that Unity 2021.3+ LTS is recommended. Update it to reflect the minimum supported version, Unity 2022.3 LTS, while also mentioning Unity 6 or later, then review the rendered guide to confirm the wording is accurate.

Written by the indexing model from the issue text.